1. Field of the Invention
The present invention relates to a drive pulse generating apparatus, and more particularly to a drive pulse generating apparatus suitable for driving a drive device using an electromechanical transducer and an appliance including the drive pulse generating apparatus and drive device.
2. Description of the Prior Art
There has been known a linear movement type actuator or a rotary type actuator constituted such that elongation and contraction displacement is caused in an electromechanical transducer, for example, a piezoelectric element for driving a member constituting a camera or other precision devices, where the elongation and contraction displacement is transmitted to a driving member and a driven member is moved via a moving member frictionally coupled to the driving member (refer to Japanese Unexamined Patent Publication No. JP-A-4-69070 and Japanese Unexamined Patent Publication No. JP-A-63-11074).
FIG. 43 shows an example of a drive device applied in driving a zoom lens mounted to a camera in which slidable fitting portions 102a and 102b of a supporter 102 for supporting a lens barrel 101 that is a driven member, are brought into frictional contact with and slidably fitted to a drive shaft 103. Further, the drive shaft 103 is supported displaceably in the axial direction by support portions 105 and 106 of a frame 107. One end of a piezoelectric element 108 displacing in the thickness direction is fixed to one end portion of the drive shaft 103 in the axial direction and other end of the piezoelectric element 108 is fixed to the frame 107, and the drive shaft 103 is displaced in the axial direction by displacement of the piezoelectric element 108 in the thickness direction.
Further, numeral 104 designates a leaf spring which is fixed to the slidable fitting portions 102a and 102b of the supporter 102 by small screws, not illustrated, from the lower side in FIG. 43. A bent portion 104a that is bent upwardly is formed at the central portion of the leaf spring 104 by which the bent portion 104a is brought into pressing contact with the drive shaft 103 for causing a suitable frictional force at the contact portion.
When wavy drive pulses each comprising a gradual rising portion and a steep falling portion successive thereto as shown by FIG. 44(a), are applied to the piezoelectric element 108 in the drive mechanism shown by FIG. 43, the piezoelectric element 108 is displaced to elongate gradually in the thickness direction at the rising portions of the drive pulses and the drive shaft 103 is moved gradually in the axial direction in a direction of an arrow marked "a".
In this case, when a frictional force exerted between the drive shaft 103 and the slidable fitting portions 102a and 102b of the supporter 102 plus a frictional force exerted between the drive shaft 103 and the bent portion 104a of the leaf spring 104 are equal to or lower than a force exerted to the drive shaft 103 by the piezoelectric element 108, the supporter 102 is moved in the arrow mark "a" direction along with the drive shaft 23 in a state of being frictionally coupled to the drive shaft 103 and the lens barrel 101 is moved in the direction designated by the arrow mark "a".
In the meantime, at the steep falling portions of the drive pulses, the piezoelectric element 108 is displaced to contract rapidly in the thickness direction and accordingly, the drive shaft 103 is rapidly moved in the axial direction in a direction opposed to the arrow marked "a". The supporter 102, supported by the drive shaft 103 at the slidable fitting portions 102a and 102b, stays at the position by surpassing the frictional force exerted between the drive shaft 103 and the slidable fitting portions 102a and 102b of the supporter 102 and the frictional force exerted between the drive shaft 103 and the bent portion 104a of the leaf spring 104 by inertia thereof and accordingly, the lens barrel 101 remains unmoved.
By continuously applying the drive pulses having the above-described waveform (trapezoidal waveform) to the piezoelectric element 108, the lens barrel 101 can be moved continuously in the direction designated by the arrow mark "a" (forward direction). In moving the lens barrel 101 in a direction opposed to the arrow marked "a" (rearward direction), the rearward movement can be achieved by applying to the piezoelectric element 108 drive pulses having a waveform comprising a steep rising portion and a gradual falling portion successive thereto as shown by FIG. 44(b).
The speed control of a drive device using such a piezoelectric element has been accomplished by a method of adjusting a frictional coupling force exerted between a driving member and a driven member frictionally coupled thereto (supporter, lens barrel and the like in the above-described example), or by a method of adjusting the amplitude of the drive pulse.
However, according to the method of adjusting the frictional coupling force exerted between the driving member and the driven member frictionally coupled thereto, a constitution for adjusting the frictional coupling force is needed, that is, in the above-described example where the bent portion 104a bent upwardly is formed at the central portion of the leaf spring 104, the bent portion 104a is brought into pressing contact with the drive shaft 103 and a pertinent frictional force is caused at the contact portion, means for adjusting the height of the bent portion 104a for adjusting the press contact force or the like is needed and accordingly, the constitution is complicated.
Further, according to the method of adjusting the amplitude of the drive pulse, as shown by FIG. 47, in a range B where the amplitude of the drive pulse is equal to or greater than a predetermined value, the amplitude of the drive pulse applied to the piezoelectric element is substantially proportional to the drive speed of the driven member frictionally coupled to the driving member and the larger the amplitude of the drive pulse, the faster the drive speed. However, in a range A where the amplitude of the drive pulse is small, there causes a dead zone where the drive speed of the driven member frictionally coupled to the driving member remains unchanged even when the drive pulse is applied to the piezoelectric element, ripples are caused in the drive speed, the driven member cannot be driven, or the drive speed cannot be adjusted finely.
Further, the speed control of a drive device for driving the lens barrel 101 or the like can be carried out by thinning drive pulses applied to the piezoelectric element. That is, when the drive speed in driving the piezoelectric element by a series of continuous drive pulses is designated by notation V, if the drive efficiency is lowered by thinning the drive pulses from the series of drive pulses by a predetermined ratio, the drive speed is lowered in accordance with the ratio of thinning by which a target drive speed Vt can be provided.
Specifically, for example, when the drive speed for driving the piezoelectric element by the series of continuous drive pulses is designated by notation V and the target speed Vt can be provided by lowering the drive efficiency to m/k, the series of continuous drive pulses is divided into groups at every number of pulses of k (for example, k=5), m (for example, m=2) of the drive pulses are left in the groups of the series of drive pulses and (k-m) pulses (5-2=3 in the above-described example) are removed, the drive efficiency is lowered to m/k (2/5=0.4 in the above-described example) by which the target speed Vt can be provided.
FIG. 45 indicates a pulse series showing the above-described example in which a series of continuous drive pulses is divided into groups each comprising 5 pulses where 2 (m=2) drive pulses are left and 3 drive pulses are removed. Further, FIG. 46 is a diagram showing an example of a relationship between the drive efficiency and the drive speed.
As mentioned above, the conventional speed control of a drive device using a piezoelectric element is carried out by thinning drive pulses, however, it has been found that according to the method of speed control, there are drawbacks explained below.
According to a drive device using a piezoelectric element, a drive shaft is reciprocated at different speeds by displacements of elongation and contraction of the piezoelectric element having different speeds by which a driven member frictionally coupled to the drive shaft is moved. When the piezoelectric element is displaced gradually, the driven member is moved along with the drive shaft by being frictionally coupled to the drive shaft and when the piezoelectric element is displaced abruptly, the drive shaft stays in a stationary state by inertia by surpassing the frictional coupling force in respect of the drive shaft. That is, the driven member is moved in a predetermined direction by repeating the moving and stationary states and the drive speed in that case indicates an average drive speed provided by the repetition of the moving and stationary states.
Therefore, when the drive pulses applied to the piezoelectric element are not thinned, although the piezoelectric element, the drive shaft and the driven member of the drive device are vibrated at the frequency of the drive pulses, the vibrational noise is prevented from being offensive to the ear by setting the frequency of the drive pulses to audio frequencies or higher and mechanical vibration is not caused so considerably.
However, when the drive pulses are thinned, firstly, large intermittent mechanical vibration having a frequency in correspondence with the frequency of thinning is caused in the piezoelectric element, the drive shaft and the driven member.
Secondary, even when the frequency of the drive pulses is set to audio frequencies or higher, when the frequency of thinning falls in the range of audio frequencies, vibrational noise offensive to the ear is caused.
Thirdly, when the ratio of thinning the drive pulses is increased in order to drive the driven member at a low speed, nonuniformity of speed is enhanced and vibrational noise offensive to the ear becomes louder.
Furthermore, according to a pulse generating apparatus for generating drive pulses, a complicated circuit is used and therefore, the following problems to be resolved are posed.
Firstly, comparatively high voltage (several tens of volts) is needed for a peak voltage of the drive pulse applied to the piezoelectric element and therefore, when a low voltage power source (several volts) such as a battery is used as a power source, a booster circuit having a complicated constitution is needed for generating high voltage.
Secondary, according to a circuit of generating drive pulses in a trapezoidal waveform where a constant current circuit, a switching circuit and a timing circuit for controlling timing are combined, a total of the device is magnified and power consumption is increased.